1. Field of Invention
The present invention generally relates to overprint compositions for coating toner-based prints that provide a number of advantages to toner-based prints, such as, for example, image permanence, thermal stability, lightfastness, and smear resistance. The invention further relates to reducing or preventing thermal cracking by assessing the degree of thermal cracking on coated toner-based prints after thermal shock.
2. Description of Related Art
In conventional methods of generating toner-based images, such as in xerographic methods, electrostatic latent images are formed on a xerographic surface by uniformly charging a charge retentive surface, such as a photoreceptor. The charged area is then selectively dissipated in a pattern of activating radiation corresponding to the original image. The latent charge pattern remaining on the surface corresponds to the area not exposed by radiation. Next, the latent charge pattern is visualized by passing the photoreceptor past one or more developer housings comprising toner, which adheres to the charge pattern by electrostatic attraction. The developed image is then fixed to the imaging surface or is transferred to a receiving substrate, such as paper, to which it is fixed by a suitable fusing technique, resulting in a xerographic print or toner-based print.
Known methods of protecting prints include adding wax to the toner for toner-based prints and applying an overprint coating to the substrate to protect the print from abrasives and provide scratch resistance, for example, for toner-based and ink-based prints. The overprint coating, often referred to as an overprint varnish or composition, is typically a liquid film coating that can be dried and/or cured. Curing is generally accomplished through drying or heating or by applying ultraviolet light or low voltage electron beams to polymerize (crosslink) the components of the overcoat. However, known overprint coating, such as those described in U.S. Pat. Nos. 4,070,262, 4,071,425, 4,072,592, 4,072,770, 4,133,909, 5,162,389, 5,800,884, 4,265,976, and 5,219,641, for example, fail to adequately protect toner-based prints.
For example, coatings specifically created to coat ink-based prints do not function effectively on toner-based prints due to a mismatch in the coefficient of thermal expansion between the coating resin and the toner resin. Thus, when the toner-based print is exposed to elevated temperatures and/or pressures, the toner expands causing the formation of hairline cracks on the surface of the print. The hairline cracks expose the substrate which, in turn, makes the cracks highly visible and degrades the quality of the image. This is a particularly important issue for automobile manuals, book covers, etc., which require the prints therein to survive high temperatures for hours at a time, yet retain a neat appearance. Similarly, known coatings that can be applied to toner-based prints do not effectively prevent or reduce toner-specific problems, such as, for example, thermal cracking and document offset.
Moreover, known coating formulations fail to protect xerographic prints from bead-up and smears caused by overwriting on the print with liquid markers. The ability to neatly overwrite without beading and smearing is vital for numerous commercial applications, such as, for example, restaurant menus and calendars.
Accordingly, a need exists for a protective composition that provides overprint coating properties including, but not limited to, thermal and light stability and smear resistance, particularly in commercial print applications. More specifically, a need exists for an overprint coating that has the ability to wet over silicone fuser oil (generally found on xerographic substrates), permit overwriting, reduce or prevent thermal cracking, reduce or prevent document offset, and protect an image from sun, heat, etc. The compositions and processes of the present invention, wherein a toner-based print is coated with a radiation curable overprint composition, satisfies this need.